In the current mobile communication environment, a 4G (4th generation) LTE (Long Term Evolution) system has been introduced, in addition to 2G and 3G systems, and various mobile communication service frequency bands coexist according to communication systems or operators and countries. In addition, the base station environment has been diversified. In order to reduce the consequent operating cost of base stations, a multi-band base station system has been adopted for implementing various communication systems with a single base station.
FIG. 1 is a diagram of a conventional multi-band base station system.
The conventional multi-band base station system includes a plurality of base stations 102 and 104 using different communication standards and frequencies, and a plurality of antennas 172 and 174 corresponding to each base station. In order to transfer RF (radio frequency) signals, DC (direct current) signals, and control signals from the respective base stations 102 and 104 to each antenna, combiners 110 and 150 and modems 120 and 140 are provided between the base stations and the antennas, and the lower modem 120 located on the base station side and the upper modem 140 located on the antenna side are connected by a coaxial feeder cable.
The upper and lower combiners 110 and 150 have L and C resonance cavities formed therein in a multistage manner in accordance with frequency characteristics to separate signals of specific bands such as, for example, 2G, 3G and 4G frequency bands or to merge these signals.
Accordingly, the combiners 110 and 150 can simultaneously support service frequency signals of various communication systems for allowing multiple base stations to share one coaxial feeder cable. Therefore, an antenna tower may use a reduced length of coaxial feeder cable to save the cost of cable installation.
Meanwhile, the control unit 130 supplies a control signal for controlling antenna line devices (ALDs) 162 and 164 such as a remote electrical tilting (RET) device, and a DC signal (DC power signal) for supplying power to the ALDs 162 and 164.
The lower modem 120 functions to combine a control signal (for example, RS485 signal) and a DC signal (DC power signal) received from the control unit 130 with an RF signal received from the lower combiner 110, and to transfer the combined signals though the coaxial feeder cable.
The upper modem 140 separates the received RF signal, control signal, and DC signal to transfer the RF signal to the antennas, and to transfer the control signal and the DC signal to the ALDs 162 and 164.
FIGS. 2A and 2B are schematic diagrams of a structure of the lower combiner of FIG. 1.
Referring to FIG. 2A, the combiner 110 includes a plurality of first ports connected to respective base stations, for example, Port 1 connected to a base station using an AWS (Advanced Wireless Service) frequency band and Port 2 connected to a base station using a frequency band of 700 MHz, and a second port (Port 3) connected to a common coaxial feeder cable.
The combiner includes RF signal paths 210 and 230 and DC bypass paths 220 and 240 which are branched from the respective ports. On the RF signal paths 210 and 230, filter units 212 and 232 are provided for performing a band pass filtering on the RF signals. The RF signals having passed through the filter units 212 and 232 are output to the second port (Port 3).
Referring to FIG. 2B, each DC bypass path is branched into a DC signal line and a control signal line through which a DC signal (DC power signal) and a control signal are transferred to the second part (Port 3), respectively. A diode is installed on the DC signal line to prevent reverse current from flowing.
Since such conventional combiners include the function of reverse current prevention, the upper combiner and the lower combiner cannot have the same structure. This inhibits a common use of combiners of the same structure for the upper link and the lower link.
In addition, with the conventional combiners, increasing the number of supported service frequency bands proportionally generates more insertion loss. Antenna Interface Standards Group (AISG), which sets standards for the control interface of antenna line devices, specifies a restriction on insertion loss. Therefore, the number of supported service frequency bands cannot be increased beyond a certain number with the conventional combiner structure.
In addition, when a multi-band base station system is implemented using conventional combiners, a separate, external modem 120 or 140 needs to be installed, which complicates the system configuration.